Hot and cold rolled steels are both used to fabricate a wide variety of goods, but each have unique properties depending on the temperature at which the steel has been worked or rolled in relation to the steel's critical temperature range, i.e., a temperature range through which the properties of the steel change dramatically. Hot rolled steel provided by steel mills is coated with a tough skin of steel oxide primarily composed of Fe.sub.3 O.sub.4, known as "mill scale," whereas cold rolled steel has a smooth surface with very few blemishes. Hot rolled steel also has more rust that is tightly bound to the surface than does cold rolled steel.
As generally described in Metals Handbook, Volume 1: Properties and Selection, 8.sup.th ed., T. Lyman, Ed. ASME 1961, p. 79, which is incorporated herein by reference: "Hot rolled sheets are suitable where normal surface oxide and minor imperfections are acceptable, and are generally used for parts where finish is of secondary importance . . . Cold rolled sheets are characterized by improved surface finish or special temper and properties." These differences are commonly taken into consideration when selecting a steel type, as well as a method for cleaning the surface prior to applying conversion coatings and paints. Conversion coating generally involves the treatment of steel to convert surface metal to a layer of insoluble inorganic crystals, thereby imparting corrosion resistance and aiding in paint adhesion. The primary conversion coatings are phosphate and chromate, the former being described below in greater detail.
There are a number of methods for cleaning iron and steel surfaces. Known methods include acid pickling, acid cleaning, alkaline descaling, salt bath descaling, brushing, and abrasive blasting or tumbling. In the automobile industry, acid cleaners have not been commercially embraced to nearly the same extent as alkaline cleaners.
Pickling and cleaning each involve surface treatment with a caustic solution. One difference between pickling and cleaning is that pickling usually involves immersion, whereas cleaning usually involves wiping or spraying. Another difference is that cleaning solutions are generally more dilute than pickling solutions. As stated in Metals Handbook, Volume 2: Heat Treating, Cleaning and Finishing, 8.sup.th ed., T. Lyman, Ed. ASME 1964, p. 340, which is incorporated herein by reference: "The distinction between acid cleaning and acid pickling is a matter of degree, and there is often some overlapping in the usage of these terms. In general, however, acid pickling refers to more severe treatment for the removal of scale from semifinished mill products, forgings or castings, whereas acid cleaning is the term most frequently used when the acid solution is employed for final or near-final preparation of metal surfaces prior to plating, painting or storage." Cleaning solutions may have acid concentrations as dilute as pH 5.5, or as concentrated as acid pickling solutions, e.g., 35 vol. percent acid or more. For example, a cleaning solution having 70 vol. percent phosphoric acid is reported in Metals Handbook, Volume 2, at p. 341.
The most common acids involved in pickling are sulfuric and hydrochloric acid, while commonly used acid cleaning agents include organic acids and weaker solutions of sulfuric and hydrochloric acid, and a variety of other mineral acids, including phosphoric acid. Dilute phosphoric acid cleaners are known to leave a light iron phosphate coating, which provides temporary resistance to corrosion and a suitable base for painting.
Acid cleaning is often carried out at room temperature, although heating is known to greatly increase efficiency. When heating is employed, the temperature of the acid is generally maintained in a range of about 140-200.degree. F.
Cold rolled steel has long been the preferred material of construction in the automobile industry because its good surface quality enables the achievement of high quality paint finishes. Moreover, automobile parts and smaller articles have been considered amenable to chemical cleaning and conversion coatings because of their relatively small size and because those skilled in the art have found that cold rolled steel does not have so much rust and mill scale as to prevent effective use of such chemical methods.
The steel frames of ships, rail cars, earth movers, bank vaults, and power transformers are unlike those of automobiles because, in addition to being much larger, such articles are primarily hot rolled, rather than cold rolled steel. The combination of larger size and degree of mill scale and rust on steel frames of ships and rail cars has led to a view that such articles are nonanalogous to cold rolled automobile frames and smaller articles with respect to pre-painting preparation techniques, which has limited the acceptance of chemical pickling and cleaning methods in the rail car industry.
Other methods for surface cleaning and preparation include power tool cleaning, water washing, and abrasive injection in water. Power tool cleaning can remove a very high degree of rust and mill scale, but like blast cleaning, power tool cleaning generates large quantities of dust, while consuming large amounts of energy and requiring frequent maintenance and replacement of worn parts. Water washing equipment requires specialized components for operating at medium to high pressure (3000 psig or higher). For example, a pressure pump, a specialized lance and a nozzle assembly are required, in addition to large volumes of water. This technique can remove loose rust but will not effectively remove tight rust or mill scale. Abrasive injection in water may provide a greater ability to remove rust and mill scale. However, abrasive injection in water raises most of the concerns of ordinary blast cleaning, in addition to consuming large volumes of water.
Due to the drawbacks of these alternative methods, mill scale and rust on large articles made of hot rolled steel has conventionally been removed by blast cleaning, a method of removing mill scale, rust, rust scale, paint or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels. Typically, a "Commercial Blast Cleaned Surface Finish" is sought, which is defined by the Steel Structures Painting Council (SSPC) as one from which all oil, grease, dirt, rust scale and foreign matter have been completely removed from the surface, and all rust, mill scale and old paint have been completely removed except for slight shadows, streaks, or discolorations caused by rust stain, mill scale oxides or slight, tight residues of paint or coating that may remain. If the surface is pitted, slight residues of rust or paint may be found in the bottom of pits. Under this definition, at least two-thirds of each square inch of surface area is free of all visible residues and the remainder is limited to the above-mentioned light discoloration, slight staining or tight residues.
Three major categories of abrasives that are used for blast cleaning include "grit," "shot," and "sand." "Grit" primarily comprises angular metallic particles, usually from cast steel that is crushed and hardened, and may be tempered or chilled white cast iron. Grit has extraordinary cutting power. "Shot" is also metallic, but comprises spherical particles or cut wire that deforms into rounded particles during use. "Sand" includes ordinary silica sand as well as a number of other nonmetallic abrasives, including natural minerals, such as garnet, dolomite, pumice, novaculite, and flint quartz, and synthetic materials, such as silicon carbide and aluminum oxide. These abrasives are suited to applications in which contamination with metals is undesirable.
In the United States, the field of rail car surface preparation is led by Trinity Industries, Thrall and National Rail Car. Each of these companies have employed grit blast cleaning for many years, due in large part to the longstanding understanding in the art that grit blast cleaning is the most upwardly scalable surface preparation method for articles having a high degree of mill scale and rust. Grit blast cleaning can obtain virtually any degree of cleaning of any size article--from Brush-off Blast Cleaning of a gas grill LP tank in accordance with standard SP7 of the SSPC (SSPC-SP7), to a White Metal Blast Cleaning of a rail car in accordance with standard SP5 of the SSPC (SSPC-SP5). Standard SSPC-SP7 for Brush-off Blast Cleaning provides:
Removal of loose mill scale, loose rust, and loose paint, to the degree hereafter specified, by the impact of abrasives propelled through nozzles or by centrifugal wheels. It is not intended that the surface shall be free of all mill scale, rust, and paint. The remaining mill scale, rust, and paint should be tight and the surface should be sufficiently abraded to provide good adhesion and bonding of paint. . . . PA1 Removal of all mill scale, rust, rust scale, paint or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels. A White Metal Blast Cleaned Surface Finish is defined as a surface with a gray-white, uniform metallic color, slightly roughened to form a suitable anchor pattern for coatings. . . .
Standard SSPC-SP7 for White Metal Blast Cleaning provides:
In addition to cleaning, steel pretreatments may also include chemical conversion coatings for imparting corrosion resistance and improving paint adhesion. Although manufacturers do not apply chemical conversion coatings to rail cars, phosphate conversion coatings are applied to properly cleaned steel surfaces in other industries. The three principle phosphate conversion coatings are iron phosphate, zinc phosphate, and manganese phosphate.
Iron phosphate conversion coatings, which are the oldest commercial phosphate coatings, are primarily used as a base for paint because they provide excellent adherence and resistance to flaking. However, the automobile industry has come to rely mainly on zinc phosphate for chemical conversion. Zinc phosphate is considered by the automobile industry to offer improved corrosion resistance and greater paint adhesion.
Phosphate conversion coatings may be applied by immersion, by wiping, or by spray application of the phosphate solutions. Immersion methods may be used for applying conversion coatings to small articles, whereas spray coating may be used for applying phosphate compositions to larger articles, such as transformer housings, which can be as high as twenty feet. Spray application of iron phosphate provides a layer of fine crystals having a bluish tint on steel surfaces that are substantially rust free. The layer usually has a thickness from 20 to 150 mg/ft.sup.2. Phosphate coatings may be dried by various types of ovens, at temperatures up to 400.degree. F.
All phosphate conversion coating compositions are acidic. Zinc phosphate can be sprayed at a pH as high as 3, whereas zinc phosphate immersion solutions are typically maintained within the pH range of 1.8-2.4. Iron phosphate immersion solutions are commonly maintained within a pH range of 3-6.
The inventor is not aware of any reported use of spray cleaning and conversion coating for preparing surfaces of large hot rolled steel articles, such as rail cars.